In the lighting industry light mixing rods are widely used to collect light from a light source and mix the collected light into a light beam. The light from the light source enter the light mixing rod through an input end and propagates through the light mixing rod and is typically reflected at the sidewalls of the mixing rod one or more times. Due to the multiple reflections the result is that the light from the light sources is mixed and the light distribution at the output end of the light mixing rod is substantially uniform. Light rods are thus used to provide a substantially uniform light distribution from a light source having a non-uniform light distribution.
Further, light mixing rods are often used to mix light from a light source having a plurality of light emitters emitting different colored light, and where the purpose of the light mixing rod is to mix the light from the different light emitters into one uniform light beam with all colors equally mixed. For instance light mixing rods are widely used to mix light from 4in1 red, green, blue, white (RGBW) light emitting diodes (LEDs), which comprises a red LED die emitting red light, a green LED die emitting green light, a blue LED die emitting blue light and a white LED die (typical phosphor based) emitting white light.
Light mixing rods can have many different shapes as known in the prior art and are typically provided as a solid body of an optical transparent material where the light inside the light mixing rod undergo total internal reflection while propagating trough the light rod. The prior art solid light mixing rods are provide in PMMA or glass. Alternatively, the light mixing rods can also be provided as a hollow body with reflective surfaces for instance in metal or glass coated with reflective material, however these are relativity expensive to produce and ineffective due to loss of light at the reflections.
The known solid light mixing rods of PMMA or glass are arrange a distance above a light emitting window of the light sources in order not to scratch or in other ways destroy the light emitting window or the entrance surface. As a consequence, an air gab is introduced between the emitting window of the light source and the entrance surface of the solid light mixing rod, and the amount of light entering the light rod are reduced, due to the transmission from the light emitting window to air and due to the transmission form air to the light mixing rod. Further, some of the light from the light source will be emitted outside the entrance surface of the light rod and thus not enter the light rod. As a consequence, a substantially amount of light is lost.
It is known that the loss of light due to the change in refractive index caused by the air gab can be reduced by providing a refractive index matching material between the light emitting window of the light source and the entrance surface of the solid light mixing rod. However, this process adds significant cost to the manufacturing process as the refractive index matching material need to be provided manually. The process of providing the refractive material is difficult and persons performing the process must be very accurate as the light sources and light rods are small (less the 1 cm2). In addition there is risk that the index matching material flows away by time, as most proper and affordable index matching materials is formed as a gel.
WO 06049801 discloses an LED package with a mechanically decoupled optical element where to decoupled optical element is in optical contact with LED die via an air gap. WO 06049801 also discloses that the optical element is optically coupled to LED die via a thin optically conducting layer that is disposed within the air gap. Examples of materials suitable for the optically conducting layer include index matching oils, and other liquids or gels with similar optical properties.
WO12083957 discloses an optical light mixer providing a homogenized and uniform light beam where the optical light mixer is formed of a solid transparent material and light enters the optical light mixer through an entrance surface and is reflected through said body to an exit surface where said light exits said optical light mixer. The optical light mixer comprises a first part formed as a pyramid frustum where the top surface of the pyramid frustum forms the entrance surface and a second part formed as a cone frustum and where the bottom surface the cone frustum forms the exit surface. The present invention relates also to an illumination device comprising a number of light sources and such optical light mixer.
DE 199 61 527 A1 discloses a light fixture where a light emitting diode is arranged in a light housing and on a circuit board arranged. A light guide is arranged in the cover of the light fixture housing and the light guide is arranged above the light emitting diode when the cover is arranged to the light fixture housing. The light fixture housing is filled with a potting compound. In one embodiment, the light emitting diode is held above a circuit board on a flexible conductor strip. The light guide is non-elastic and causes the flexible conductor strip to bend towards the circuit board when the cover is attached to the light fixture housing. In another embodiment, the light guide is a permanently elastic light guide which is compressed against the light emitting diode, when the cover is arranged to the light fixture housing. There is a risk that air, potting compound or other materials will be arrange between the light emitting diode and the light guide when arranging the light guide above the light emitting diode. In addition, it is difficult to arrange the light guide in an accurate position above the light emitting diode.